> Lots of vent fans in houses tend to not actually vent anywhere, or they're covered up with insulation.
I do a lot of house repairs. I don't think I've ever seen a bathroom fan that goes outside except in mobile homes. They just vent into the attic. Same with many kitchen fans. They should all go to the outside otherwise they could cause moisture issues in the future.
Good bathroom ventilation is important. Controlling humidity is a big thing, but also properly ventilating things like acetone or hairspray or other things along with bad odors is important.